feature: Debugging utilities + more user-friendly error listing

[tecc]

This PR introduces some more user-friendly debugging features that should help alleviate the need of opening issues here on the repository. The main feature that is added is the debug page (or debug section of the settings page), which allows Notea to display issues it has detected whilst running. For example, if the server was configured improperly (say the store access key was not set), Notea will redirect you to the /debug page and tell you that the store configuration was not configured properly.

Due to its implementation in this PR, the feature will not work as well as it can when using serverless/function-based architectures (i.e. hosting on Vercel or similar services) instead of having a central server. It is also very rudimentary - it currently only handles configuration-related problems (albeit it is very easy to add more issues), and is not translated.

TODO:

• [x] Detailed explanations for all currently handled issues
• [x] Proper handling for improper store configurations

[tecc]

Right now, it's looking ready for merging. This is the current version of the /debug page, with some misconfigurations. (You can view this for yourself over at https://notea-phy69jqq9-notea-org.vercel.app)